Re: Session-ID proposal

Shel Kaphan (
Thu, 10 Aug 1995 14:14:31 -0700

Dave Kristol writes:
> (Koen Holtman) wrote (on www-talk):
> > Dave Kristol:
> > [....]
> > >
> >
> > This proposal is not clear enough about caching. Specifically:
> >
> > is the session-id header in the request part of the cache key for the
> > entity in the response?
> No. In section 2.3 I said:
> Similarly, a caching proxy must pass back to the requestor any
> Session-ID response header it receives, but it must not cache that
> header as part of its cache state.
> >
> > If it is, this means that almost no meaningful caching is possible for
> > services using session-id, even if 99% if the entities in the session
> > (inline pictures, product description pages) do not depend on the session
> > state.
> Yes, exactly.

However, please note that the "side channel" of state information that
flows both directions and bypasses proxy and user-agent caches, even
if the resources themselves are cached, is not cheap. Setting up and
tearing down the TCP connections is a nontrivial fraction of the cost
of retrieving a small resource (but I admit: I don't have numbers) --
especially html files, as opposed to large graphics or audio media files.
And those media files are typically cacheable anyhow. Even on systems
where URLs contain session-IDs, the URLs for the media files usually
need not, and so they're cacheable.